Extended Data Fig. 1: Conceptual linkages between solubilisation, respiration, remineralisation, and microbially-mediated fragmentation of particles intercepted by and incubated within C-RESPIRE and POC flux attenuation.

Representation of how C flux attenuation within the inner chamber of C-RESPIRE (left panel) driven by bacterial solubilisation of particles using exo-enzymes is linked to measured bacterially-driven respiration (as a decrease in O₂ (lower right panel) converted to C using a respiratory quotient (see main text)). Other microbes, including heterotrophic microflagellates (upper right panel), contribute to particle fragmentation and degradation⁵⁶ (see also the Supplementary Video 1 from ref. ⁹⁶). The oxygen optode time-series during the multi-day incubation phase is characterised by a typical decrease in the rate of MR toward the end of the incubation (i.e., after ~24–48 h; grey area on the oxygen time-series) (see also ref. ⁹), indicative of the onset of substrate limitation for microbes. Kamalanathan et al.⁹⁷ reported a robust relationship between oceanic POC stocks and C-acquiring exoenzymes that revealed the links between the C needed for bacterial physiology from POC substrates (i.e., polysaccharide depolymerization). This coupling between solubilisation and respiration is further supported by the low accumulation rate of dissolved organic C within the C-RESPIRE inner chamber during incubations (Extended Data Table 4). There is growing evidence of strong links between exoenzyme activity, environmental sensing by attached bacteria^19,98, microbial biomass⁹⁷, and C utilisation⁹⁹. Bacterial growth is correlated with respiration¹⁰⁰, and bacterial respiration is the major driver (~90%) of the bacterial carbon demand (total amount of C required to sustain the heterotrophic metabolism, i.e., production plus respiration) based on a compilation of >200 paired bacterial production and respiration from 0–200 m depth from the US-JGOFS open ocean global study⁴³. Hence bacterial respiration closely tracks bacterial carbon demand under natural conditions⁴³.